Conversions are the heartbeat of a website. Without some form of conversion a site is pointless.
Conversions can be filling in a contact form, downloading information, buying product or reading content. They are the reason the website exists.
Of course there are some actions that we ‘want to happen’ that we can’t measure directly from the website; for example a phone call. But there are ways of recording these such as dedicated trackable numbers or just pen and paper lists.
If each conversion represents someone doing the thing you want then it stands to reason that you want to maximise this and this is what conversion optimisation is all about.
There are many reasons why conversions can be poor. Some are purely related to the position of elements on the web page, some are due to a lack of trust elements, some are due to distractions that take visitors away from the page.
Whilst experience is often used to suggest revisions to improve conversions. The only real way for high volume sites is to test side by side with the original version using tools like Google Experiments. However for low volumes we tend to rely on experience and data.